As telecommunication systems have evolved from 10-gigabit-per-second (Gb/s) data transfer rates to 40 Gb/s and now toward 100 Gb/s, it has been desirable to develop more sophisticated modulation schemes. As a result, over the past decades, modulation of optical signals has gradually changed from simple amplitude on-off keying to more advanced modulation formats, such as phase modulation. At transmission rates of 40 Gb/s and higher, phase modulation schemes such as differential phase-shift keying (DPSK) and differential quadrature phase-shift keying (DQPSK) are now typically used. Such advanced modulation formats require an optical phase demodulation device relying on interference between neighboring bits before light detection, also known as a delay line interferometer (DLI). Examples of typical configurations for the optical demodulator suitable for DPSK and DQPSK modulation formats are illustrated in FIG. 1 (PRIOR ART) and FIG. 2 (PRIOR ART), respectively. Alternatively, a 90-degree optical hybrid mixer, such as a 2×4 multimode interference (MMI) coupler, can be used in order to simplify the DQPSK demodulator, as shown in FIG. 3 (PRIOR ART) [see, e.g., K. Voigt et al., “SOI based 2×2 and 4×4 waveguide couplers—Evolution from DPSK to DQPSK”, IEEE International Conference on Group IV Photonics (GFP), paper ThBS (2008)].
While discrete components were initially used to build DPSK and DQPSK optical receivers, an interest currently exists in providing integrated solutions. Indeed, as data transfer rates rise toward 100 Gb/s, coherent communication involving polarization multiplexing further adds to the complexity at the transmitter and receiver sides. In particular, a need arises for accurately controlling the optical paths between the optical demodulation device and the detection assembly.
For at least this reason, it is now admitted that advanced modulation formats in general, and DPSK and DQPSK in particular, call for photonic integration. Over the past decade, integrated photonics has made much progress in implementing optical and electro-optical devices for use in various technological applications in fields such as optical telecommunications and signal processing. Integrated photonics relies on optical waveguides to implement devices such as optical couplers, transmitters and receivers, wavelength multiplexers and demultiplexers, and polarization splitters and rotators. In addition to improve functionality, integrated solutions are typically more cost effective and more compact. Component size is generally an important parameter in telecommunication systems, so that the introduction of new components is typically followed by an effort toward both cost and size reduction.
Among existing integrated photonic technologies, submicron silicon-on-insulator (SOI) technology provides the advantage of maximized compactness, which is made possible by the high refractive index contrast between the silicon core and silica cladding of the SOI waveguides. This enables propagation of highly confined optical modes and allows scaling integrated photonic devices down to submicron level. For example, SOI waveguides allow designing circuits with radius of curvature as small as 3 microns, which is about two orders of magnitude smaller than what can be achieved with other integration technologies. Moreover, silicon-based integrated photonics is compatible with silicon electronics and standard complementary metal-oxide-semiconductor (CMOS) fabrication methods. However, submicron SOI circuits generally support the propagation of only one polarization mode, typically a transverse electric (TE) mode, thereby complicating the realization of polarization-insensitive devices, which is highly desirable for telecommunication receivers. Single-polarization optical receivers based on submicron SOI circuits have been reported [see, e.g., S. Faralli et al., “25 Gbaud DQPSK Receiver Integrated on the Hybrid Silicon Platform”, IEEE International Conference on Group IV Photonics (GFP), paper FA4 (2011)], but their commercial interest is limited.
A polarization diversity scheme, in which two orthogonal polarizations coming out of an input port are routed along different paths, has been proposed for a proper management of the polarization [W. Bogaerts et al., “A polarization-diversity wavelength duplexer circuit in silicon-on-insulator photonic wires”, Opt. Expr. vol. 15 no. 4, p. 1567 (2007)]. The two polarizations may be separated using a polarization beam splitter such as a two-dimensional (2D) surface grating coupler [D. Taillaert, et al., “A Compact Two-Dimensional Grating Coupler Used as a Polarization Splitter”, IEEE Photon. Technol. Lett. vol. 15, no. 9, p. 1249 (2003)]. Such a polarization splitting has been used to produce a polarization-division multiplexed receiver based on submicron SOI circuits [C. R. Doerr and L. Chen, “Monolithic PDM-DQPSK receiver in silicon”, Proc. of ECOC 2010, paper PD3-6 (2010)].
Photonic-integrated circuits based on large waveguides (i.e. with a width and height in the range between about 2 and 4 micrometers) supporting both polarizations have also been used to implement integrated receivers [M. Kroh et al., “Hybrid Integrated 40 Gb/s DPSK Receiver on SOI”, Proc. of OFC 2009, paper OMK3 (2009); and L. Zimmermann et al., “Towards Silicon on Insulator DQPSK Demodulators”, Proc. of OFC 2010, paper OThB3 (2010)]. However, in addition to sacrificing the high compactness provided by submicron waveguides, polarization-insensitivity remains a challenge as both polarizations propagate into the waveguides of the device with slightly different velocities, thereby leading to a polarization-dependent operation. This difference in velocity results in a polarization-dependent frequency shift (PDFS), which corresponds to a spectral shift between the respective spectra of optical elements, such as interferometers, of an optical device supporting the propagation of modes with different states of polarization. This phenomenon may originate, for example, from fabrication inaccuracies, material non-uniformities, such as in thickness or refractive index, and thermal fluctuations. An optical receiver is generally considered to provide a polarization-insensitive operation when the PDFS is small, typically when the PDFS less than about 1 GHz.
To ensure a polarization-insensitive operation of an optical receiver based on a polarization diversity approach where the input signal is split into two orthogonal polarization components, the circuits associated with the two polarizations generally need to be identical. This poses significant fabrication challenges in terms of precisely controlling the waveguide dimensions and positioning, which may not achievable even using state-of-the-art CMOS fabrication techniques. Bogaerts et al. addressed this issue by propagating both polarizations through the same circuit but in opposite direction, using an arrayed waveguide grating (AWG) designed such that both polarizations could use the same set of delay lines. However, an approach such as disclosed by Bogaerts et al. and consisting in using a same device for both polarizations but in reverse propagation direction is suitable only for very specific applications. It is not suitable for example in a DPSK or a DQPSK optical receiver.
There therefore exists a need in the art for a compact and integrated optical receiver configured for demodulating phase-modulated optical signals, which exhibits polarization-insensitive operation and reduced sensitivity to non-idealities and uncertainties including fabrication errors and material non-uniformities